Method and apparatus for the diagnosis of natural vibrations in a mechatronic system

ABSTRACT

A method for the diagnosis of natural vibrations in a mechatronic system, includes at least one rigid body (WT 1 , WT 2 ), which is moved relative to another rigid body (S) by means of at least one numerically-controlled drive, whereby the natural vibrations are caused by the drive (A 1 , A 2 ) and are detected by sensors (WM 1 , WM 2 , N 1 , N 2 , BA 1 , BA 2 ,) internal or external to the system. The drives (A 1 , A 2 ) can be used as a system-internal, vibration-generator, so-called rotatory shakers, by superimposing white noise on a constant speed for the drive, or by wobbling the frequencies.

BACKGROUND OF THE INVENTION

The invention relates to a method for the diagnosis of naturalvibrations in a mechatronic system, for example a machine tool,packaging machine and other manufacturing machines, e.g. in the form ofa robot. These mechatronic systems normally include a plaurality ofmachine elements, including drives, which cause unwanted vibrationsduring operation of the system. Each one of these vibrations resultsfrom a combination of forced vibrations, caused by external forces orunbalances for example, as well as from natural vibrations which areencountered when one or more of the resonances of the machine areexcited. These vibrations adversely affect the quality of productsproduced by the mechatronic system, so that ways to substantially reducethese vibrations are searched for.

Computation and visualization of these vibrations therefore gainincreasingly importance. This is true in particular for the area ofvibration diagnosis. This refers hereby essentially to the effect whichthe machine elements of the mentioned complex mechatronic systems haveon the dynamic behavior of the overall system.

The purpose of vibration diagnosis is to make the technical designer ofa mechatronic system understand as clearly as possible the vibrationbehavior of the system in order to give him a tool to use constructivemeasures and/or select materials for inhibiting vibration.

For diagnosis of natural vibrations of a mechatronic system, it isnecessary to excite it. Conventional methods use external vibrationexciters, so-called translatory shakers, which are attached at differentpoints of a rigid body of a mechatronic system. Vibrations of differentfrequencies are impressed upon the rigid body via these shakers. Atdifferent areas of the rigid body, especially at its corner points,sensors, for example by means of displacement measuring systems,accelerometers, pressure gauges, etc., are used to detect the naturalvibrations.

A mathematical model of the vibration behavior of the overall system canbe formed on the basis of the measured values. As all system componentsare, however, interconnected, a system of coupled differential equationsis obtained which are converted by means of model analysis, i.e. bymeans of an algorithmic mathematical uncoupling, into different scalarequations, so that information can be provided about the naturalfrequency, the attenuation, and form of the vibration for each mode of anatural vibration.

On the basis of these equations, a model of the mechatronic system canbe simulated and visualized by means of a simple wire-frame model forexample. Such wire-frame models are conventional and are constituted bycorner points where sensors are typically attached in such a manner thata maximum yield of information is obtainer. The design engineer thus isgiven insight into the system dynamics and can take vibration-absorbingconstructive measures through modification of the oscillation parametersthat are now known.

The process set forth above for generating natural vibrations usingshakers external to the system has the drawback that the drives andtheir part components are not included in the vibration diagnosis andthe simulation.

Conventional software packets for model analysis identify and representvibration modes in such a manner that either translatory or rotatorydegrees of freedom are related to. A simultaneous visualization of bothtypes of degrees of freedom is achieved in practice by so including therotatory degrees of freedom into the holistic visualization thatrotatory vibration modes are stimulated using movements of a rigid bodyin relation to a fixed point. However, this approach does naturally nottake into account the compliance that continuously exists betweenmechanical structure and drives.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a method for thediagnosis of natural vibrations which allows inclusion of the entirertyof the system components, including the drives.

This object is attained by a method for the diagnosis of naturalvibrations of a mechatronic system, wherein the system includes at leastone rigid body which is movable relative to another rigid body by atleast one numerically-controlled drive, wherein the natural vibrationsare excited by the drive and detected by sensors. Thus, the need for asystem-external shaker is eliminated which is no longer required becausethe drives assume their task. Through use of the drives as additionalrotatory shakers, the eleectro-mechanic properties of the drives canenter into the integrated mechatronic modelling so that the afore-statedcompliances between mechanical structure and drives can be betterascertained.

According to an advantageous configuration of the method, the drive,which runs, for example, at a constant speed, is superimposed by whitenoise. White noise is used to excite the natural vibrations of thesystem.

According to a further advantageous configuration of the method, thenatural vibrations of the system can also be excited by superimposingthe speed at which the drive runs with sinusoidal vibrations of varyingfrequency. This method of known conventionally as “wobbling”.

It is especially advantageous that the described method allows adetermination of the natural vibrations by means of system-internalsensors, such as, for example, displacement measuring systems,acceleration sensors, speed sensors, pressure measuring systems, etc. Asa consequence, the vibration diagnosis can be carried out completelyautarchic, i.e. with system-internal means, so long as the numericalcontrol of the mechatronic system includes a respective hardware moduleand/or software module.

According to a further advantageous configuration of the invention, themethod can be enhanced by attaching to the mechatronic system additionalsensors, like, for example, acceleration pick-ups These additionalsensors may be attached, when needed, or remain permanently on thesystem. When provisions are made, for example, to carry out repeatedvibration diagnosis during the service life of the mechatronic system,because of the assumption, for example, that the natural vibrationschange as a result of material fatigue and/or other reasons, theadditional sensors may remain also on the system for simplification ofthese further diagnoses.

The method can be carried out advantageously with an apparatus for thediagnosis of natural vibrations of a numerically-controlled mechatronicsystem, wherein the apparatus includes at least one rigid body which ismoved relative to another rigid body by at least onenumerically-controlled drive, wherein the numeric control of the systemincludes a hardware module and/or software module, wherein the hardwaremodule and/or software module includes means for excitation of naturalvibrations of the system by the numerically-controlled drives as well asmeans for processing the natural vibrations detected by the sensors.

According to an advantageous configuration of the apparatus, the modulecan be connected as external module to the numeric control. The numericcontrol would hereby only include the terminals for the module so thatthe module can be connected to the numeric control, when needed, i.e.for execution of the vibration diagnosis. As a result, compexity andcosts of the numeric control can be kept low. The vibration diagnosiscould then be carried out “on order” by a service expert, for example,by the service department of the supplier of the numeric control. Inthis case, there would even be no need for the customer to pay for theexpenses for the module. The separate module may, however, also bedesigned in such a universal manner, as to be suitable for a widevariety of machines and numeric controls so that a single module can beused for the diagnosis of various mechatronic systems.

The module may, however, also form an integral part of the numericcontrols. This would simplify the repeated diagnosis of vibration of amechatronic system.

BRIEF DESCRIPTION OF THE DRAWING

An exemplary embodiment of the invention will now be described in moredetail with reference to the drawing, in which:

FIG. 1: shows the wire-frame model or a machine tool;

FIGS. 2, 3 and 4: show various vibrations models.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows the wire-frame model of a machine tool with a pedestal S, atool table arranged on the pedestal and including a table element WT1,movable in Y-direction, and a table element movable in X-direction. Thetable elements WT1 and WT2 are operated by the drives A1 and A2 and thepertaining spindles. The color boundary of the colors black and grey ofthe spindles designate the connection areas between the respective tableelement WT1, WT2 and the spindles.

The asymmetric illustration is caused by the generation of thewire-frame model which is defined by the corner points of the structuralparts or components being viewed. In the present example, these cornerpoints are so disposed as to be asymmetric. The shape of the wire-framemodel should reflect the present geometry in a best possible mannerthrough simplest basic elements. Moreover, the areas are selected ascorner points, such as, e.g., E1 and E2, that also yield meaningfulmeasuring information.

Attached to the components of the machine tool represented by thewire-frame model are displacement measuring systems WM1 and WM2 assensors. These displacement measuring systems normally belong to thesystem-internal components of the actual machine tool. These additionaldisplacement measuring systems may, of course, be provided, if requiredfor the diagnosis of the natural vibrations. The Figure depictssymbolically acceleration pick-ups BA1 and BA2 as further sensors forthe diagnosis of the natural vibrations. Of course, further additionalsensors may be used, such as, for example, pressure gauges, etc. Thespeed sensors N1, N2 of the drives A1, A2, which are also present in thesystem, are also used as further sensors for the diagnosis of thenatural vibrations.

The machine tool is controlled by a numeric control NC which inlcudesfor each drive an axis module AM1, AM2. In concert with the axis modulesAM1, AM2, the numeric control regulates the drives A1, A2 in aconventional manner.

The numeric control NC is connected with a diagnosis module DM forvibration diagnosis. The diagnosis module DM may be a separate modulewhich is implemented on a data processing machine, for example a laptop.It may, however, also form an integral part of the numeric control NC—asindicated also by the broken line.

Diagnosis of the natural vibrations is realized by operating the drivesA1, A2 at constant speed nc, as illustrated in the graphicalrepresentation of the block diagram of the diagnistic module DM. Thisconstant speed nc is superimposed with white noise—symbolized in theFigure by the sine curve. White noise contains all frequency portionsand thus also those that excite the system to produce naturalvibrations. The constant speed nc could also be superimposed withvarying frequencies, whereby these varying frequencies are appliedsuccessively (wobbled).

The natural vibrations of the mechatronic system are thereby reliablyexcited and can be detected by the displacement measuring systems WM1,WM2, the acceleration pick-ups BA1, BA2, and the speed sensors N1, N2.

As the natural vibrations are excited by the drives A1, A2, also thecomponents of these drives, which also contributre to the vibrationbehavior of the overall system, are considered. As a consequence, thevibration diagnosis determines also the continuously present compliancesbetween the machine structure and the drives.

Relaxation oscillations can be detected by suitably placing the sensorsas far as possible also at the corner points of the rigid body. Sensorsat the corner points E1 and E2 allow detection of, for examplerelaxation oscillations of the tool table WT2 about the y-axis. The sameis true for the sensors (not shown here) at further corner points, e.g.at the corner points of the tool table WT1.

The results gained from the vibrations diagnosis allow a visualizationof the vibration behavior of the mechatronic system by a wire-framemodel, for example. With the assistance of the modal analysis, theanimation of the rotatory vibration modes in the space is gained fromthe combination of a three-dimensional linearized translation with aone-dimension rotation about a single axis. The vibration diagnosis issignificantly simplfied by examining the effect of all machine elementspartaking in the respective vibration mode and demonstrating it to thecustomer. An advantage that cannot be underestimated is hereby affordedfor the acceptance of the measured results and possibly of the simulatedresults that have been suited to the measurements because clearerinformation can be made about the influence by the machine structure andthe drives upon the dynamics of the mechatronic system.

FIGS. 2, 3 and 4 show some vibration modes by way of example at varyingfrequencies. The deformations are depicted by broken line so that theycan be better visualized, and the difference compared to the undeformedstate becomes clearer. The individual elements of the respective Figureare known from FIG. 1. FIG. 2 shows a vibration mode at 56 Hertz, FIG. 3shows a vibration mode at 81 Hertz, and FIG. 4 shows a vibration mode at106 Hertz.

Each vibration mode on conventional mechatronic machines containstranslatory as well as rotatory degrees of freedom whose separateconsideration is of little value in view of a constant linkage betweentranslatory and rotatory degrees of freedom. The procedure describedherein fully considers this linkage from the mechatronic viewpoint bythe simulatenous animation of all geometric basic elements, such asparallelepiped and cylinder, which represent the wire-frame model. As aresult, reliable information is generated about the influence ofrotatory and translatory degrees of freedom on the resultant vibrationmode, when the drives are coupled in a compliant way to the mechanicalstructure.

Shakers, external to the system and acting translatory, can easily beintegrated into the developed modal analysis tool for generatingadditional information.

1. A method for ascertaining natural vibrations in a mechatronic system,comprising the steps of: moving at least one rigid body relative toanother rigid body by means of at least one numerically-controlleddrive; and detecting natural vibrations, generated by the drive, bymeans of sensors, wherein the natural vibrations are generated by thedrive by superimposing white noise upon a rotation speed of the drive.2. The method of claim 1, wherein the sensors in the detecting step aresystem sensors.
 3. The method of claim 1, wherein the detecting stepincludes additional sensors.
 4. A method for ascertaining naturalvibrations in a mechatronic system, comprising the steps of: moving atleast one rigid body relative to another rigid body by means of at leastone numerically-controlled drive; and detecting natural vibrations,generated by the drive, by means of sensors, wherein the naturalvibrations are generated by the drive by superimposing sinusoidaloscillations upon a rotation speed of the drive.
 5. The method of claim4, wherein the sensors in the detecting step are system sensors.
 6. Themethod of claim 4, wherein the detecting step includes additionalsensors.
 7. A method for ascertaining natural vibrations in amechatronic system, comprising the steps of: operating anumerically-controlled drive at a constant rotation speed; exciting thedrive to generate natural vibrations; moving at least one rigid bodyrelative to another rigid body by means of the drive; and detecting thenatural vibrations generated by the drive by means of sensors, whereinthe exciting step includes superimposing the rotation speed of the drivewith white noise.
 8. A method for ascertaining natural vibrations in amechatronic system, comprising the steps of: operating anumerically-controlled drive at a constant rotation speed; exciting thedrive to generate natural vibrations; moving at least one rigid bodyrelative to another rigid body by means of the drive; and detecting thenatural vibrations generated by the drive by means of sensors, whereinthe exciting step includes superimposing the rotation speed of the drivewith sinusoidal oscillations.
 9. Apparatus for the diagnosis of naturalvibrations of a numerically-controlled mechatronic system, comprising:at least one rigid body; a numeric control having at least one moduleselected from the group consisting of hardware module and softwaremodule; at least one drive operated by the numeric control for movingthe rigid body in relation to another rigid body, wherein the moduleincludes means for superimposing a rotation speed of the drive withwhite noise to generate natural vibrations; a plurality of sensors fordetecting the natural vibrations; and processing means for processingthe natural vibrations detected by the sensors.
 10. The apparatus ofclaim 9, wherein the module is connectable to the numeric control. 11.The apparatus of claim 9, wherein the module is an integral part of thenumeric control.
 12. Apparatus for the diagnosis of natural vibrationsof a numerically-controlled mechatronic system, comprising: at least onerigid body; a numeric control having at least one module selected fromthe group consisting of hardware module and software module; at leastone drive operated by the numeric control for moving the rigid body inrelation to another rigid body, wherein the module includes means forsuperimposing a rotation speed of the drive with sinusoidal oscillationsto generate natural vibrations; a plurality of sensors for detecting thenatural vibrations; and processing means for processing the naturalvibrations detected by the sensors.
 13. The apparatus of claim 12,wherein the module is connectable to the numeric control.
 14. Theapparatus of claim 12, wherein the module is an integral part of thenumeric control.